Application Note

Boiler and Feedwater Pump Efficiency Optimization

▪ Industry: Power ▪ Product: DPharp Pressure Transmitter, EJA-E and EJX-A series

▪ Introduction The primary function of a utility boiler is to convert water into steam With oil-burning and gas-burning boiler effeciencies over 90%, power to be used by a steam turbine/generator in producing electricity. plants are examining all associated processes and controls for efficiency The boiler consists of a furnace, where air and fuel are combined and improvements. Between 1 and 3% of the gross work produced by a boiler burned to produce combustion gases, and a feedwater tube system, is used to pump feedwater. One method of improving overall efficiency the contents of which are heated by these gases. The tubes are is by controlling feedwater pump speed to save on pump power. connected to the steam drum, where the generated water vapor is drawn. In larger utility boilers, if superheated steam (low vapor ▪Application saturation) is to be generated, the steam through the drum is passed Feedwater Flow Control - In electrical power plants, demineralized through superheater tubes, which are also exposed to combustion feedwater is pumped to the boiler tube header. The feedwater flow is gases. Boiler drum pressures can reach 2800 psi with temperatures regulated by either controlling the pump speed by using a process over 680° F. Small to intermediate size boilers can reach drum pressures control valve. The normal effect of pure water corrosion-erosion (or between 800 and 900 psi at temperatures of only 520° F if superheated cavitation) that causes premature seat failure within the valve is steam is desired. Small to intermediate size boilers are only being amplified because velocities of 10+ fps can be reached. considered for this application note.

Figure 1: Typical Boiler Drum/Feedwater System PAG506-01.a

ThePAG indicator-506 1stkit will Edition allow for04/2013 any of these three orientations. www.yokogawa.com/us LH.012111.a Page 2 of 2

Figure 2: Boiler Optmization: Pressure vs. Steam Flow PAG506-02.a

The added cost of frequent valve replacement makes utilizing a bench calibrated and installed into a high static pressure line without feedwater pump the preferred method of flow control. Controlling re-zeroing. DPharp is unaffected by these pressure surges due to its pressure by varying the pump speed saves on overall pump power superior overpressure protection. The transmitter performance specs usage. are guaranteed over two years. Pump Speed Control - Controlling pump speed is accomplished by The power plant's control system can use the high pressure differential variable-speed electric drives or by close-coupling a steam turbine measurement to automatically set the feedwater rate depending on to the pump. By definition, cogeneration power plants produce the boiler load demand. At lower demand periods, less steam is used both electricity and excess steam where turbine exhuast is used as to power the steam turbine driven pump and less feedwater is a heat source. A steam turbine driven feedwater pump is most often consumed. Significant savings are realized through steam applied because of the abundance of available steam. conservation and by lowered maintenance costs with the use of Boiler Optimization- With a boiler's firing rate held constant, the DPharp transmitter. feedwater pressure and flow affect boiler pressure. As electricity demands change, so do associated boiler loads. When more electricity needs to be generated, the boiler produces more (Also see Application Note on Boiler Drum Level Measurement steam, therefore feedwater flow must be increased to the boiler. PAG-505) Significant savings are realized if boiler drum pressure and feedwater discharge pressure are held to a constant differential. This can be accomplished by accurately measuring this pressure differential and controlling the pump speed and feedwater flowrate accordingly. In this example, boiler drum pressures can vary between 800 and 900 psi depending upon the load, (See Figure 2). The feedwater pump discharge pressures can vary between 875 and 1000 psi depending upon boiler demand. Differential pressure between the two points can be up to 200 psi, or even higher in system upset conditions. This differential pressure must be accurately measured, without regard to the 800 psi and above changing boiler drum static pressure.

▪Yokogawa Solution DPharp's EJA110E and EJX110A "V" range capsule can measure differential pressures up to 2000 psi with the same ±0.055% and ±0.040%, respectively, of span reference accuracy of lower range capsules. In this common application example, ranging the EJA110E V range transmitter 0 to 300 psi allows for the measurement of both boiler pressure and feedwater pump discharge pressure. During pump start-up severe pressure spikes of 500+ psi can occur which can cause shifts in zero. Many competitors claim re-zeroing their transmitter at static line pressure minimizes these zero shifts but this task may not be practical under fluctuating pressure conditions. The DPharp transmitter can be

ThePAG indicator-506 1stkit will Edition allow for04/2013 any of these three orientations. www.yokogawa.com/us LH.012111.a